UNAIDS estimates that there are 35-40 million people infected with HIV. Heterosexual transmission of HIV continues to spread this epidemic to an estimated 3-4 million people a year. Development of acceptable female controlled strategies, including microbicides, is a high priority for national and international agencies. The SIV/rhesus macaque vaginal transmission model has been used to test anti-HIV microbicides. We propose to refine the model by developing a multiple low-dose SIV inoculation that more closely mimics the biology of HIV transmission than the current high-dose model. We hypothesize that both the number of target cells infected (dose) and genetic complexity of the viral population that infects those cells, determine the outcome of vaginal SIV exposure. We will determine the mechanism of action for select microbicides, using the high dose IVAG challenge model to rigorously test the ability of a compound to prevent IVAG SIV transmission and acquisition of systemic infection. Once developed, we will use the multiple, low-dose SIV vaginal challenge model to: 1) determine if a microbicide that prevents infection in the high-dose model, allows acquisition of local occult infection with the potential for progression to systemic infection; 2) assess the effect of chronic microbicide use on vaginal pH, bacterial flora, cervicovaginal inflammation and SIV/HIV target cell populations; and 3) define the effect of chronic microbicide use and repeated SIV exposure on genital tract innate and adaptive immunity. It is expected that these studies will allow us to determine the mechanism of action of microbicides in-vivo and test our hypothesis regarding the role of dose and viral population complexity in vaginal SIV transmission. Further, these studies will determine if individuals that consistently and correctly use of microbicides in the face of repeated viral exposure develop anti-viral immune responses or local infection.